Closed loop daylight harvesting light control system having auto-calibration

ABSTRACT

An apparatus and method for enabling an automatic calibration sequence for a light control system having a daylight harvesting scheme is disclosed herein. An ambient light sensor connects to a detection circuit for detecting the amount of ambient light within a given zone. A microprocessor connects between the detection circuit and a dimming circuit for providing control to initiate dimming and to start the auto-calibration sequence. Responsive to the amount of ambient light detected, the dimming circuit controls the power supplied to a plurality of electrical loads. A storage unit connects to the microprocessor for storing minimum light levels assessed during the auto-calibration sequence wherein the ambient light levels are monitored for a predetermined amount of time to determine the lowest level of ambient light generated for the purpose of setting and storing a target voltage level associated with such.

This application claims the benefit of the filing date of a provisionalapplication having Ser. No. 60/677,920 which was filed on May 5, 2005.

FIELD OF THE INVENTION

The present invention relates to light control switches, and, moreparticularly, to a daylight harvesting light control system.

BACKGROUND OF THE INVENTION

Daylight harvesting is an available lighting strategy designed to reduceexcessive internal light levels during peak consumption hours, whereinexternal light sources, such as daylight, substitute for interiorelectrical lighting. For example, in an office setting, each work areamust at all times be provided with a desired level of light determinedbased upon the tasks performed in the area or zone. Lighting, however,is generally installed by size and number sufficient to provide theminimum light level under the assumption that no other light sources areavailable in the interior space. Yet, during varying times of the day,other light sources may illuminate the interior space such that thelevel of light present is excessive. Therefore, the use of interiorlighting at the same level of intensity becomes a waste of energy.

Specifically, during the day, sunlight may enter through windows andskylights. When these external light sources are present, the presetbrightness of interior lighting is not necessary since these externallight sources provide some or all of the minimum light level required.Daylight harvesting eliminates the excessive level of intensity ofinterior lighting, conserving as much as 84% of the energy required tolight a facility at the minimum light level. As such, bright sunlight isenabled to provide up to 100% of illumination during midday, when energycosts are highest.

Daylight harvesting also enables a constant level of light on worksurfaces to avoid moments when the external light sources provide anexcessive amount of light, resulting in periods of glare. In thealternative, when light levels are low (i.e. when clouds roll in ornighttime falls), daylight harvesting maintains this constant level oflight by continuously increasing and decreasing the power provided tothe internal lighting. This practice enables the worker to resolveimages with ease. As a result, eyestrain is avoided; and health andproductivity are promoted.

Conventional technology for implementing daylight harvesting techniquesincorporates the use of digital photo-sensors to detect light levels,wherein the digital photo-sensor is connected to a dimmer controlcircuit to automatically adjust the output level of electric lightingfor promotion of a lighting balance. Dimmer control circuits, asimplemented with respect to daylight harvesting, gradually increase ordecrease interior lighting in response to photocell measurement ofambient light levels.

In general, dimmer control systems are widely used in indoor lighting toprovide a softer feel and more controllable illumination experience ascompared to on/off lighting. It is desirable to provide dimmer controlsystems for fluorescent as well as for incandescent lighting.Conventional dimmer control circuits include on/off switching andup/down power controls. Further, a microprocessor may be incorporatedwithin a dimmer control circuit to provide control for various power-up,power-down and fade in/out functions. Rather than use a variableresistor type rheostat which wastes power and generates heat at lowillumination levels, modern dimming control circuits employ phaseregulation, in which the power circuit is switched on at a time delayfollowing a zero-crossing of the AC sine wave input until the end ofeach half cycle, in an effort to supply a variable level of power to thelighting load. For dimming control in fluorescent lamps, a ballast witha controlled low voltage (0-10 V) input is desired.

When commissioning a lighting system installation that employs daylightharvesting techniques, the target voltage level for the minimum level oflight or the desired steady state light level must be established forthe light sensor which conventionally is a standard 0-10 volt photocell.This target voltage level is generally set manually. Accordingly, thisapproach requires a user to monitor the system after sunset and recordthe lowest value read. Thereafter, the user is required to set thedesired steady state light level according to this value.

This approach, however, is not cost efficient nor convenient since itemploys the use of staff or personnel after hours.

Thus, a need exists for a better calibration technique than has beendescribed above for determining the target voltage level of a lightsensor included within a light control system having a daylightharvesting feature.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of establishing and settingthe target voltage level light sensor incorporated within a lightcontrol system having daylight harvesting feature, the present inventionteaches an apparatus and method for enabling a daylight harvestingcontrol device to automatically calibrate the light sensor within thelight control system by setting a target voltage level required tomaintain an acceptable minimum level of light for an interior spaceilluminated therewith.

The light control system in accordance with the present inventionincludes an ambient light sensor connected to a detection circuit fordetecting the amount of ambient light within a given zone. Amicroprocessor, having an auto-calibration subroutine, connects betweenthe detection circuit and a dimming circuit for providing control toinitiate dimming. The dimming circuit connects to electrical loads tocontrol the power supplied these electrical loads based on the amount ofambient light detected. A storage unit connects to the microprocessorfor storing the target voltage level and minimum light levels.

The automated calibration method in accordance with the presentinvention includes a step where all electrical loads are forced to fullillumination for a defined period of time, such as a 24 hour timeperiod. In another step, all control device inputs to each electricalload are disabled during this period of time. In yet another step, alight sensor is monitored during the period of time and the light levelsensed by the light sensor is detected. At this point, each detectedlight level is compared with a previously stored light level todetermine which light level is the lower. A target voltage level isgenerated based upon the lowest light level. Finally, a storage unitstores this target signal level.

In an embodiment of the light control system of the present invention,the dimming circuit may include a triac coupled to a zero crossingdetector to employ phase regulation, in which the power circuit isswitched ON after a time delay following a zero-crossing of the AC sinewave input until the end of each half cycle, in an effort to supply avariable level of power to the plurality of electrical loads. Thedimming circuit may also be configured to provide an output in the range0-10 V to control a ballast for use with a fluorescent lamp.

Advantages of this design include but are not limited to a closed loopdaylight harvesting photocell target auto calibration method having ahigh performance, simple, and cost effective process.

These and other features and advantages of the present invention will beunderstood upon consideration of the following detailed description ofthe invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numbers indicate like features and wherein:

FIG. 1 shows a flow chart of the auto-calibration technique inaccordance with the present invention; and

FIG. 2 shows a block diagram of the daylight harvesting light controlsystem having auto-calibration in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set for the herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

FIGS. 1 and 2 show a method and apparatus for enabling a daylightharvesting control device to automatically calibrate the light sensorwithin the light control system by setting the target voltage levelrequired to maintain an acceptable minimum level of light in theinterior space that is being illuminated.

Referring to FIG. 1, in step 102 all loads are forced to fullillumination. Next, in step 104, all control devices inputs to theplurality of electrical loads are disabled during a defined period oftime for auto-calibration. In one embodiment, this time period may beset to a 24 hour period to enable the light control system to monitorthe ambient light level at all times of the day. Step 106 employsmonitoring of the light sensor. The light level is detected in step 108.In step 110, the sensed light level is compared with a previously storedlight level. In step 112, it is determined which light level is thelower. A target voltage is generated for the lower light level in step114. Finally, in step 116 the lower light level and the target voltagelevel are stored.

The automated calibration method in accordance with the presentinvention takes advantage of the fact that the amount of light that theinstalled plurality of electrical loads provide at full brightness,where no external light is present such as that which comes from the sunthrough the windows, is typically the desired design level. The minimumbrightness of the electrical loads is used as a comparison for theamount of light needed at full brightness. This typically occurs atnight after work crews have left a given site where the light controlsystem is installed.

Referring to FIG. 2, the light control system 200 in accordance with thepresent invention includes an ambient light sensor 202 connected to adetection circuit 204 for detecting the amount of ambient light within agiven zone. A microprocessor 222, within a control device 220, connectsbetween the detection circuit 204 and a dimming circuit 230 forproviding control to initiate dimming. The dimming circuit 230 connectsto a plurality of electrical loads 240 to control the power supplied tothe plurality of electrical loads based on the amount of ambient lightdetected. Further, the control device 220 may include a set ofuser-actuated switches 224 (S₁, S₂, S₃ . . . S_(n)) for user controlthat overrides the daylight harvesting feature.

In operation, the ambient light sensor 202 generates a first signal inresponse to being exposed to radiation. Detection circuit 204 generatesa second signal from the first signal. Storage unit 206 is coupled tomicroprocessor 222 and is used to store light levels and to retrievelight levels. Microprocessor 222 receives the second signal fromdetection circuit 204—and compares it to a stored light level—to controlthe plurality of electrical loads. Dimming circuitry 230 connectsbetween the microprocessor 222 and the electrical loads for increasingand decreasing the illumination of the electrical loads based on thecontrol signals generated by microprocessor 222.

Microprocessor 222 includes the auto-calibration subroutine. During themethod of automated calibration in accordance with the presentinvention, the microprocessor 222 monitors the light level sensed bylight sensor 202 for the defined period of time. During initiation ofthe automated calibration process, microprocessor 222 generates a thirdsignal and sends the third signal to the dimming circuitry 230 to forcethe plurality of electrical loads 240 to full illumination. Afterwards,the microprocessor 222 disables control of the plurality of electricalloads and sets a target signal level equal to the second signal at thebeginning of the defined period of time; that is the initial targetlevel is set to the illumination level detected prior to setting all theloads to full illumination. The storage unit 206 stores the targetsignal level. Continuously, microprocessor 222 monitors the detectedlight level from the detection circuit for the defined period of timeand compares a current light level (obtained from the second signal)with the previous detected light level. Microprocessor determines whichof the two levels is a lower level. This is done continuously until thedefined time period lapses at which point a lowest level will have beenfound for the defined period based on the above procedure. At thispoint, microprocessor 222 generates a new target level based on theselected lowest level. The selected lowest level and the new targetlevel are then stored in the storage unit 206.

In an embodiment of the light control system in accordance with thepresent invention, dimming circuit 230 may include a triac 232 coupledto a zero crossing detector 234 to employ phase regulation, in which thepower circuit is switched ON at a time delay following a zero-crossingof the AC sine wave input until the end of each half cycle, in an effortto supply a variable level of power to the plurality of electricalloads. The electrical loads may also include a fluorescent lamp with aballast, with the dimming circuit configured to provide a variable lowvoltage (0-10 V) output to the ballast.

The light control system in accordance with the present invention mayfurther include apparatus for initiating the auto-calibration function,such as a DIP switch (not shown).

Advantages of this design include but are not limited to a closed loopdaylight harvesting photocell target auto calibration method having ahigh performance, simple, and cost effective process.

Those of skill in the art will recognize that the physical location ofthe elements presented in FIG. 2 can be moved or relocated whileretaining the function described above.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference.

All the features disclosed in this specification (including anyaccompanying claims, abstract and drawings) may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

1. A method of auto-calibration for setting the target voltage level ofa light sensor included within a daylight harvesting controller,comprising the steps of: a. forcing all attached electrical loads tofull illumination for a defined period of time; b. disabling all controldevice inputs to each electrical load during the defined time period; c.monitoring the light sensor during the defined time period; d. detectingthe light level sensed by the light sensor; e. comparing each detectedlight level with a previously stored light level; f. determining whichlight level is the lowest light level; g. generating a target voltagelevel based upon the lowest light level; h. storing the lowest lightlevel and the target voltage level; and i. repeating steps c through h.2. The method as recited in claim 1, wherein the defined period of timeis 24 hours.
 3. A light control system for controlling the brightness ofa plurality of electrical loads, comprising: an ambient light sensorthat outputs a first signal in response to being exposed to radiationfor sensing the ambient light level; a detection circuit coupled to thelight sensor to generate a second signal from the first signal; astorage unit coupled to receive the second signal; a microprocessor,having an auto-calibration subroutine, the microprocessor coupled toreceive the second signal from the detection circuit to control theplurality of electrical loads; and a dimming circuitry unit coupledbetween the microprocessor and the plurality of electrical loads forincreasing and decreasing the illumination of the plurality ofelectrical loads; wherein, during the activation of the auto-calibrationsubroutine for a predetermined period of time, the microprocessorgenerates a third signal and sends the third signal to the dimmingcircuitry to force the plurality of electrical loads to fullillumination and disables control of the plurality of electrical loads,the microprocessor sets a target signal level equal to the second signalat the beginning of the predetermined period of time and stores thetarget signal level in the storage unit, the microprocessor monitors thesecond signal for the predetermined period of time and compares thesecond signal with the stored target signal level to determine which isa lower signal, the microprocessor sets the target signal level to equalthe lower signal and stores the target signal level in the storage unit.4. The light control system as recited in claim 3, wherein the dimmingcircuitry comprises: a triode alternating current switch (triac); and azero crossing detector coupled to the triac to employ phase regulation.5. The light control system as recited in claim 3, further comprising: adip switch coupled to the microprocessor to activate theauto-calibration subroutine.
 6. The light control system as recited inclaim 3, wherein the dimming circuitry is configured to provide anoutput in the range 0-10 V.
 7. The light control system as recited inclaim 3, wherein the electrical loads comprise a fluorescent lightsource.
 8. The light control system as recited in claim 7, wherein theelectrical loads further comprise a ballast.
 9. The light control systemas recited in claim 8, wherein the dimming circuitry is configured toprovide an output in the range 0-10 V for controlling the ballast.
 10. Amethod of determining a target light level for a light harvestingoperation, the method comprising: forcing a lighting load in a space toa defined level during an auto-calibration period; monitoring the lightlevel in the space during the auto-calibration period; determining thetarget light level by comparing a light level monitored during theauto-calibration period to other light levels monitored during theauto-calibration period; and saving the target light level for a lightharvesting operation in the space.
 11. The method of claim 10 whereinthe defined level comprises a design level.
 12. The method of claim 10wherein the defined level comprises a maximum level.
 13. The method ofclaim 10 wherein the target light level comprises the minimum lightlevel monitored during the auto-calibration period.
 14. The method ofclaim 10 wherein the auto-calibration period includes a time at whichthe external light entering the space is a minimum.
 15. The method ofclaim 10 wherein: forcing a lighting load in a space to a defined levelduring an auto-calibration period comprises activating at least onelighting fixture to illuminate an area at a highest level achievable bythe at least one lighting fixture; monitoring the light level in thespace during the auto-calibration period comprises monitoring anillumination level of the area over a predetermined period of time;determining the target light level comprises determining a lowestillumination level occurring during the predetermined period of time;and the target illumination level comprises the lowest illuminationlevel.
 16. The method of claim 15 wherein the predetermined period oftime is twenty-four (24) hours.
 17. The method of claim 15 wherein:monitoring an illumination level of the area over a predetermined periodof time comprises periodically sampling the illumination level; andsaving the target light level for a light harvesting operation in thespace comprises storing the lowest illumination level detected at asample.
 18. The method of claim 15 wherein: monitoring an illuminationlevel of the area over a predetermined period of time comprisescontinuously measuring the illumination level; and saving the targetlight level for a light harvesting operation in the space comprisesstoring the lowest illumination level detected.
 19. The method of claim15, further comprising: disabling a dimming control to preventalteration of the lighting level supplied by the at least one lightingfixture during the predetermined period of time.
 20. The method of claim15 wherein the method is performed automatically.
 21. The method ofclaim 10 wherein the method is performed automatically.